Add support for using an alternate cache location

[tfoote]

LIke apt-get's conffile or -c

[trainman419]

I'm proposing to fix this in #312 . Please review and comment there.

[Aposhian]

Is this now supported with the option:

  -c SOURCES_CACHE_DIR, --sources-cache-dir=SOURCES_CACHE_DIR
                        Override /home/adam/.ros/rosdep/sources.cache

so can this ticket be closed?

[nuclearsandwich]

@Aposhian as it turns out this option has not been fully implemented. There's a recent discussion here: #845

[peci1]

See #908, which handles not only the sources cache, but also the meta cache location.